In the previous ground solar post, we described deploying a solar power system after Hurricane Matthew to keep our freezers and refrigerators running. In the post on solar panel principles, we discussed important issues to consider when selecting solar panels. Once you’ve selected a solar panel model to create your solar power array, it is time to consider the next piece of the solar puzzle, the combiner.
As discussed previously, a solar array is composed of multiple strings of panels connected in parallel. The job of a combiner, which looks like a breaker panel, is to perform this parallel connection. Here again is our ground array in action:
The white box in the middle (it is actually gray, but just looks white in this photo) is our combiner. Here we are using three strings of three panels each. Each panel in a string is connected in series, and that series connection leads to the combiner. Arranged this way, there is plenty of room to expand to six strings, if needed, with an additional string in the gap to the lower left and one each upper and lower right.
In this photo, we have installed two breakers, of a possible six. The left breaker has been wired, while the right breaker has not. The pink wire leading to the bottom of the breaker on the left is connected to the positive terminal of the solar panel string, while its corresponding negative terminal is connected to the terminal block on the left via a black wire. Both of these wires are 10 AWG, and are a special kind of wire, known as PV wire, especially designed for exposure to the sun and weather. All of your leads between solar panels and the combiner should be made with PV wire.
The second pair of wires feeds power from the combiner back to the other system components located inside our industrial building. In the photo above and the perspective shot to the right, the positive pink wire is tied into the distribution plate at the top, while the negative black wire is connected to the shared negative terminal block on the left. Notice that I have used the word negative instead of ground. The remaining terminal block, on the right side of the combiner, is the actual ground connection. This would be used to provide a specific grounding connection to all the panel frames and the combiner. A third ground wire would feed into the inside components to complete the grounding function. Here we have not used a separate ground wire.
Using a combiner box, each string can be isolated from the array simply by throwing its respective breaker. This provides many conveniences, one of which is that you don’t have to connect a string directly to a load, leading to a nasty spark. You may recall from previous articles that the MC4 connectors used in many solar systems are easily damaged if connected live, and that solar panels are always live. Combiner breakers fix that problem. The proper sequence for connecting a string to a combiner is to:
1. Ensure the breakers for all strings are off and no strings are connected to the panel while the breaker cover is open.
2. Prepare a pair of lead PV wires, one red and one black, with MC4 connectors on one end, and stripped for the combiner panel on the other.
3. Insert the combiner end of the PV wires into the panel through a knock-out hole in the bottom of the combiner. Be sure to use a rubber grommet in each knockout hole to protect the PV wire from the sharp metal edge of the knockout.
4. Connect the positive (red) PV wire to the bottom of the breaker, and the negative (black) PV wire to the shared negative terminal on the left.
5. Connect all the solar panels in the string in series.
6. Connect the combiner lead wires to the remaining connectors on the solar panel string.
7. The breaker can now be thrown to energize the string at the panel.
You may be tempted to use AC breakers and panels in place of a combiner box. You must not do this, as AC breakers are not capable of breaking DC arcs. The breakers used in these combiners are special DC breakers which are capable of breaking high-voltage, high-current DC arcs without damage.
You will also note that we have used a 6-breaker combiner (in this case a Midnite Solar MNPV6) instead of a 3-breaker combiner (MNPV3). Expect to pay a little more than $100 for a 6-breaker combiner, and a little less than $100 for a 3-breaker combiner. While a 3-breaker combiner is a little less expensive than a 6-breaker combiner, the price difference isn’t enough to justify ruling out the 6-breaker model. Imagine if one of the panels was broken, and you needed to change your 3×3 array into a 2×4 array. In this case, a 6-breaker panel would allow you to install one of your spare breakers and field that fourth array. Otherwise, you would not be able to take advantage of the remaining two panels in your crippled string, leaving you with only six panels producing power.
It is important to pick a combiner brand and stick with that brand throughout. We like the Midnite Solar brand for our combiners and breakers. The only negative we have to say about this brand is that the black plastic breaker cover is a little loosey-goosey, but it does the job when it is in place. Also, always choose breakers from the same manufacturer as the combiner. Although some breakers can be interchanged, we do not recommend it. Expect to pay about $20 each for breakers.
For sizing the breakers in your system, read the solar panel manufacturer’s datasheet for that model. This datasheet will usually specify the correct breaker amperage, sometimes listed as a series fuse rating. In our example, we used 265 watt Canadian Solar panels. The datasheet recommended a 15 amp breaker for this model. If the datasheet does not contain a recommendation, multiply the short-circuit current (at maximum illumination) by 1.44, then round up to the next available breaker size. Our 265 watt panels are rated for 9.2 amps. 9.2 x 1.44 is 13.248, so we would round this up to 15 amps. Since we are using the Midnite Solar MNPV series panels, we need to use the Midnite Solar MNEPV series breakers, or in this case, a MNEPV15.
The combiner is the perfect place to install a surge arrestor. A sample surge arrestor is shown to the right. These are designed to conveniently install in a knockout in a combiner panel, and then wire directly into the relevant terminal strips. Since we didn’t expect any lighting to hit our panels, we didn’t use one in this set up. However, if we had intended that these be mounted permanently, such as to the top of a building or out in a field, a surge arrestor is an essential part of the combiner system. Had we used one, the model shown is a MNSPD115 from, you guessed it, Midnite Solar. Expect to pay just over $100 for each of these.
So far we have seen the panels in a solar array and a combiner. Each array feeds a single combiner, one breaker per string. All the panels in an array should be of the same model and rating, as we have discussed in a previous article. In a permanent installation, each combiner would have at least one surge arrestor. A combiner would then typically feed a single charger. If your charger is very large, and your panels are small compared to it, more than one combiner might feed your charger. But, a more cost effective option would be to move up to a larger combiner, such as a 12-breaker MNPV12, which will cost much less than two 6-breaker combiners. We will discuss the charger side of things in a future article, and refine what large versus small means, as well as how to size all of these voltages and currents to make things match.
In addition to the combiner components, you will also need PV wire, MC4 connectors (or whatever connector your panel uses), cable lugs, and crimpers for both the MC4 connectors and lugs. We’ll discuss these in a future article, also.
Practical Tips and Safety
The first practical tip is that setting all of this equipment up for the first time is more time-consuming that you might initially imagine. After all, you’re just plugging in a few wires, right? We assure you that the first time you see the spark that panels of this size are capable of making, and that the inside of the combiner box contains tens of amps at over 100 volts (about 113 volts for three of these 265 watt panels in series), and no off switch upstream of the combiner breakers, your zeal for speed will fade quickly. At a minimum, making one simple mistake of plugging a string into itself, completing a circuit, will pop the MC4 connector, requiring you to replace both ends.
Similarly, make sure you do all of your wiring in the box before you connect your strings to the PV lead wires. You do not want to be doing anything inside the box with any power at all going in or out. Eventually, you’ll connect the downstream side of the combiner to a charger. Although, in principle, the charger is isolated from the input when off, a permanent installation would see another, high-current breaker panel on the inside installation upstream of the charger in case the wiring became damaged and the outside combiner was inaccessible. This would be a perfect place for a 3-breaker combiner with a larger breaker, such as a 50-amp MNEPV50 or 63-amp MNEPV63 (a full 6-breaker combiner can easily produce 60 or so amps at full sun). There are also other options for this single-breaker role that we’ll discuss later.
Because it took so long to wire everything on the first day, we nearly missed the entire day of solar generation. Once the combiner was wired the first time, however, bringing it in at night and deploying it in the morning was a snap. We disconnected the wiring at the strings, disconnected the lead wire from the panel, and brought in the resulting octopus intact. Be sure to check the internal wiring when deploying it the next morning.
In the sections which follow, we provide Amazon links for typical items described in this article. For convenience, we have grouped these items by category.
Midnite Solar Photovoltaic Combiner – 3-Position, Model MNPV3. A 3-breaker panel is the bargain basement panel.
Midnite Solar Photovoltaic Combiner – 6-Position, Model MNPV6. While a 3-breaker combiner might be a little less expensive, we think that a 6-breaker panel is a good investment in flexibility.
MidNite Solar Photovoltaic DC Circuit Breaker – 15 Amp, Model MNEPV15. These are inexpensive enough, compared to the other system components, to get a few extra to round out the system in case you need to reconfigure your arrays after damage.
Midnite Solar Lightning Arrestor Device 150VDC MNSPD115. These may seem a little pricey at first, but each one will protect thousands of dollars worth of downstream equipment the first time it is needed.
You now know how to combine the power from several solar panel strings in a much safer way than simply tying their cables into a common point. Using your combiner, you can isolate strings for maintenance without taking down your entire solar system.
In upcoming articles, we’ll continue expanding this system toward the goal of generating useful power. Until then, be sure to visit the Off-Grid Solar Design Guide put together by one of our sponsors, SoftBaugh. That guide is chock-full of additional tips and solar-specific calculators useful for designing a complete off-grid solar capability.